In mine roof and wall support systems, a threaded reinforcing rod or rebar is embedded into the rock and a nut or threaded fastener is tightened against the rock face to consolidate forces in the rock and prevent or control ground movement. Typically, the rebar or reinforcing rod comprises a four to eight foot length of steel which is threaded at one end. The rebar is inserted into a bore hole drilled into the rock so that the threaded end projects outwardly beyond the rock face to permit the threaded coupling of the nut thereto.
U.S. Pat. No. 4,618,291 to Wright, which issued Oct. 21, 1986, discloses a preferred method of securing the rebar in a bore hole by the use of one or more resin cartridges which are inserted into the bore in advance of the rebar. After the resin cartridges are inserted, the rebar is pushed into the bore hole and driven through the resin cartridges causing the resin to mix and securely retain the rebar when set.
To enhance mixing of the resin, it is known to rotate the rebar about its longitudinal axis. One method of rotating the rebar involves positioning a cast steel dome nut over the threaded end of the rebar, and then rotating the nut with a power winch or socket. Conventional cast dome nuts typically are formed having a threaded socket which extends into a domed end portion characterized by an end opening. The domed end portion is formed with a thickness such that its engagement with the rebar prevents further movement of the dome nut onto the threaded end under initial torque forces, with the result that the rebar rotates together with the turning of the dome nut. As the resin sets, resistance to the rotation of the rebar increases. The result is that as the rotational torque forces applied to the nut exceed a critical minimum force, the domed end portion of the nut splits from the opening by the contact pressure forces of the rebar thereagainst, allowing the nut to be tightened along the rod against the rock face.
Conventional dome nuts suffer the disadvantage that the domed end configuration necessitates manufacture by expensive and labour intensive steel casting processes. The requirement of cast manufacturing further results in a dome nut which is typically of inferior quality steel as compared to the higher alloy steels used in machined steel nuts, making cast dome nuts more succeptable to mechanical failure.
The steel cast construction of dome nuts make their use in conventional mine support systems better adapted for use in coal mines or soft rock applications. In these environments, it is not unusual to tension a reinforcing rod using torques forces of 100 Ft. Lbs. or more, and to such an extent that the rock or coal seam actually compresses. This procedure may involve installing a reinforcing rod in a bore hole, allowing the resin to fully set, and then tightening the dome nut on the rod so that its threaded end is actually drawn out of the bore hole by its threaded engagement with the nut. Conventional mine roof support systems suffer the disadvantage that they are poorly suited for hard rock applications in which virtually no physical compression of the rock occurs. Some pneumatic drill equipment such as jacklegs and stoppers used in hard rock applications have difficulty producing sufficient torques to break the dome nut.
A further disadvantage with dome nuts is that, as a result of casting, it is difficult to produce nuts with consistent physical properties. In addition, to facilitate removal of the completed dome nut from a casting mould, cast dome nuts typically are manufactured with tapering opposing sides. Typically the opposing sides of the gripping portion of cast dome nuts taper inwardly towards each other in the direction of the domed end a distance of two or more millimetres. The inability to effectively produce cast dome nuts with substantially parallel sides adversely affects the gripping and twisting of the nuts with conventional wrench tools, and may lead to the nuts jamming within power winches and sockets which are used to drive the nuts onto the rebar.
Because of difficulties in casting dome nuts with their domed end portions, conventional dome nuts have a domed end which has a steel thickness of 2 mm or more. Cast dome nuts are therefore used primarily in high torque application where the torque has exceeded 100 Ft. Lbs. or more. In one attempt to manufacture a lower torque dome nut, U.S. Pat. No. 5,282,698 to Wright et al. discloses a cast dome nut in which the domed end portion is provided with a split-wall construction. Disadvantages with split dome nuts, however, exist in that in addition to difficulties in casting, split dome nuts are manufactured by casting a straight nut, and then mechanically crimping the curved end into a dome portion. As well as difficulties in ensuring consistent manufacture, mechanical crimping produces added stresses, with the result that the dome nuts do not evenly distort under consistent minimum torques. As a result, deformation of the cast domed end cannot be precisely controlled and may occur over a range of torque forces varying by 15 Ft. Lbs. or more.
The applicant has appreciated a preferred roof support system which provides for the installation of a rebar which when installed, projects two or more inches from the bore hole. With this configuration, the nut may be tightly secured against a reinforcing plate, or the rock itself, to prevent falls of ground. The projecting end portion of the rebar may thereafter be used to secure mesh screens, and/or hardware such as push-on plates used to secure screening, sill nuts and the like by the application of a second coupling nut in a second stage procedure. Conventional cast dome nuts are poorly suited for this purpose. In particular, by their operation the domed end portion splits and deforms by the action of its movement against the threaded end portion of the rebar. The contact forces between the domed end of conventional dome nuts against the external rod threads result in the scoring of the threads, effectively preventing the easy attachment of secondary components. Further, upon deformation, conventional dome nuts present a torn or split cast metal shard which extends axially from the edge of the nut 1 cm or more. Depending on the length of the rebar projecting from the bore, this may physically prevent the subsequent placement of a screen and/or hardware over the rebar to a position flush with the dome nut, and the use of a further nut used to attach the screen.